Phosgene is one of the important organic intermediates, and can be used in the fields of agrichemicals, medicines, engineering plastics, polyurethanes, military uses etc. It is particularly the important raw material for producing isocyanates and polycarbonates.
Phosgene is synthesized by using carbon monoxide and chlorine gas as the raw materials, activated carbon as catalyst in the industry. The reaction equation is showed as formula (1):CO+Cl2=COCl2ΔH=−31.6 kcal/mol  (1)High quality of phosgene is primarily provided for high-end product use. For example, for phosgene used to prepare polycarbonates, the presence of free chlorine has problems such as adverse effects on water washing and severe equipment corrosion, thus at most 50 ppm free chlorine is allowed to be contained in said phosgene; and the presence of carbon tetrachloride impurity has problems such as adverse effects in the color quality of product, thus at most 50 ppm carbon tetrachloride is allowed to be contained in said phosgene.
It can be seen from the above formula that the reaction is a very strong exothermic process, and the calories produced in the production of 1 ton phosgene are more than 300,000 kcal. In the current industry, the part of heat is transferred by chlorobenzene and circulating water, and is naturally dissipated. As such, the considerable amount of reaction heat is wasted, and the high equipment and utility cost is additionally needed. Only low-grade steam is produced even though the reaction heat can be utilized. The above-said reaction can be rapidly conducted under the action of a catalyst at ambient temperature, and releases a large amount of heat. The released heat can further accelerate the reaction speed. If the reaction speed is further accelerated, more heat will be released, which leads to a vicious cycle.
At present, the catalyst for phosgene synthesis used in the industrial fixed bed reactor is columnar activated carbon particles. The mode of catalyst packing is primarily the traditional sectional filling (or packing) mode of “inert agent+active catalyst+inert agent”. The traditional filling method not only fails to lower the reaction rate and the rate of heat release near the feed inlet to prevent temperature runaway happening, but also it may decrease the effective utilization of the active catalyst. Therefore, for the phosgene synthesis reaction, the inlet temperature of the fixed bed reactor may rise rapidly, and the temperature of the hottest spot may reach 550˜700° C. Since the content of free chlorine reduced rapidly in this period, the reaction rate will lower sharply, causing the reaction temperature to decrease rapidly. Thus, the fixed bed reactor would have a big change in the axial temperature gradient, resulting in the catalytic performance of the overall catalyst bed decreased, the uneven temperature of the heat transfer medium moved upwardly in the shell-side axial direction and the equipment material waste. And at such a high reaction temperature, long-term running will result in carbonization, pulverization of the activated carbon catalyst, so that eventually most activated carbon catalyst loses.
Since there are a large number of irregular open pores and closed pores in the interior of the activated carbon catalyst, the heat transfer effect of the activated carbon is very poor. Therefore, the radial temperature difference in the activated carbon catalyst bed layer is very big, and for a reaction tube having the inner diameter of 50 mm, the radial temperature difference may reach 300˜400° C. As such, in the course of the reaction, the failure in the control of feeding, heat removing and the like would be very easy to cause a sharp temperature rise of the catalyst bed layer, which accelerates burning, pulverization of the catalyst. Further, the burning of the catalyst would lead to lowering of the pressure drop across the bed and make more material enter into the area where the lowering of the pressure drop took place, and therefore the reaction would be further out of control, which might result in a serious safety accident.
At present, the industrial apparatus of phosgene synthesis mainly use those having low to medium-boiling point such as water, salt-free water and so on as the heat transferring medium. The heat transferring medium removes large amount of heat by circulating in the shell-side of the fixing bed reactor. The process of using the high boiling point mediums for producing vapor by vaporizing or producing steam directly by vaporizing the boiling water to remove the reaction heat and eliminate the reaction hot spots of the fixing bed reactor shall be employed. See for example EP134506, U.S. Pat. No. 4,231,959, CN200510093948.7 etc. Only Bayer Corporation used the above-said process in the industry up to now. However, the steam by-produced in the process has low quality, and the lifetime of the activated carbon catalyst is short. This is because the activated carbon catalyst has high heat resistance, such that the overall reaction bed has high temperature and heat can not be removed. Thus, if the problem of high temperature gradient in the catalyst bed, especially the problem of the high radial temperature gradient, can not be solved, the heat produced in the reaction will be difficult to be comprehensively utilized.
Meanwhile, it needs to ensure that the heat transfer coefficient is as large as possible to dissipate heat safely and uniformly, such that the conversion rate of free chlorine in the exit is maintained. If the heat transfer medium is water, it needs to solve the safety problem of leakage of the heat transfer medium to the reaction region due to the local high temperature corrosion damage, otherwise, this will affect stable operation of the apparatus adversely. If the heat transfer medium is organics, the heat transfer effect is poor, and it needs to solve the problems such as carbonization of the heat transfer medium in the shell-side at high temperature to cause clogging, difficulty of cleaning the resulting carbides and so on, otherwise this will affect stable operation of the apparatus.
As described above, the prior art methods for preparing phosgene by the catalytic reaction of carbon monoxide and chlorine gas mainly have the problems as follows: serious carbonization and pulverization of the existing activated carbon catalysts, short life time, large radial and axial temperature difference, low quality of phosgene and the by-produced steam, failure to long-term stable operation, high maintenance cost, poor safety etc. Therefore, there is a need to find a new production method to replace the existing process.